Posted by Composite Analytica on April 10, 2007 at 14:17:30:
In Reply to: CO2 Tire Inflation or Nitrogen Tire Inflation posted by Jerry H on April 08, 2007 at 05:10:12:
It is true; "storing" of CO2 gas in tyres is not an effective way of solving the global warming issue...And this is not because bicycle tyres are much smaller than SUV tyres ;-) The solubility of CO2 in the rubber is high. Moreover the diffusion coefficicient of Carbon Dioxide gas is far from ideal: the diffusivity is rather high because of slight plasticizing (further decrease of glass transition temperature of polyisobutylene rubber, enhancing the free volume available for diffusion) of the rubber.
Regarding the advantage of 100% Nitrogen in tyres, we want to inform you on the following. The fact that an ordinary air mixture would contain more and more Nitrogen in time because of the selectivity of the "membrane" was also raised by Mr. W.T. Bolk from the Netherlands. (reason for Mr. Bolk to write a letter was the publication of the assessment of tyre pumping /inflation by pure nitrogen in polyisobutylene tyre composite in the dutch newspaper "De Telegraaf", click here for the technical assessment regarding diffusion, chemical and mechanical properties of polyisobutylene rubber under the exposure of nitrogen and air under pressure).
The above would be the case if the partial pressures of Nitrogen and Oxygen of the mixture in the tyre would be equal. For example, if we would have a total over pressure of 3 bar; and N2 would have a partial pressure 1.5 bar; O2 1.5 bar partial pressure. Then, the rate of Oxygen loss would be higher than the loss of Nitrogen. Hence, the mixture in the tyre would contain more and more Nitrogen.
However, the partial pressure of Nitrogen in Air is almost a 4 times higher than Oxygen. So, although the diffusion rate of Nitrogen is slower, the driving force, caused by the partial pressure, is higher. In total, the loss of Oxygen is almost the same as the loss of Nitrogen from an air mixture permeating through the tyre! Please be aware that this is coincidence, and only valid for an air mixture and this kind of rubber.
If we replace Oxygen by Nitrogen, we can take advantage from the lower diffusion rate and relative lower solubility of Nitrogen gas in Butyl Rubber. Click here for a description of the permeation case regarding the above diffusion subject.